mac80111: aes_gcm: clean up ieee80211_aes_gcm_key_setup_encrypt()

This code is written using an anti-pattern called "success handling"
which makes it hard to read, especially if you are used to normal kernel
style.  It should instead be written as a list of directives in a row
with branches for error handling.

(Basically copied from Dan's previous patch for CCM)
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/aes_gcm.c

@@ -80,11 +80,15 @@ struct crypto_aead *ieee80211_aes_gcm_key_setup_encrypt(const u8 key[],
 		return tfm;
 
 	err = crypto_aead_setkey(tfm, key, key_len);
-	if (!err)
+	if (err)
+		goto free_aead;
 	err = crypto_aead_setauthsize(tfm, IEEE80211_GCMP_MIC_LEN);
-	if (!err)
+	if (err)
+		goto free_aead;
+
 	return tfm;
 
+free_aead:
 	crypto_free_aead(tfm);
 	return ERR_PTR(err);
 }